1. Field of the Invention
The present invention relates to an electromagnetic clutch for selective transmission of a rotating driving force, which can, for example, be utilized for selective operation of a compressor in an air conditioning system for an automobile.
2. Description of Related Art
Known in the prior art is an electromagnetic clutch which comprises a stator containing therein an electromagnetic coil, a rotor made of a magnetizing material and defining an annular opening into which the stator is inserted so that a free rotation of the rotor is allowed, an armature having a frictional surface facing a frictional surface of the rotor, a hub connected to a rotating machine such as a compressor in a refrigerating circuit for an air conditioning system for an automobile, and plate springs for connecting the armature and the hub for creating a force for holding the armature separate from the rotor so as to obtain a gap between the faced frictional surfaces of the rotor and armature, thereby preventing the rotating movement of the rotor from being transmitted to the hub. When the electromagnetic coil is energized, a closed electromagnetic field is created between the coil, the rotor and the armature, so that the armature is attracted to the rotor, which allows the rotating movement of the rotor to be transmitted to the hub.
In the prior art, the rotor has, at its outer periphery adjacent its clutch face, a stepped portion to increase the area of the clutch face, or, if no such stepped portion is provided, the thickness of the outer ring of the rotor is increased. However, these prior arts clutches are not effective for obtaining a desired freedom to select a location to connect a pulley to the rotor, while maintaining a low resistance to the flow of the magnetic flux.